Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque magnetic random access memory (STT-MRAM). STT-MRAM utilizes magnetic junctions written at least in part by a current driven through the magnetic junction. A spin polarized current driven through the magnetic junction exerts a spin torque on the magnetic moments in the magnetic junction. As a result, layer(s) having magnetic moments that are responsive to the spin torque may be switched to a desired state.
For example, FIG. 1 depicts a conventional dual magnetic tunneling junction (MTJ) 10 as it may be used in a conventional STT-MRAM. The conventional dual MTJ 10 typically includes a first conventional pinned layer 12, a first conventional tunneling barrier layer 14, a conventional free layer 16, a second conventional tunneling barrier 18, and a second conventional pinned layer 20. The conventional tunneling barrier layers 14 and 18 are nonmagnetic and are typically a thin insulator such as MgO.
The conventional pinned layers 12 and 20 and the conventional free layer 16 are magnetic. The magnetic moment 13 of the conventional pinned layer 12 is fixed, or pinned, in a particular direction. Although depicted as a simple (single) layer, the conventional pinned layers 12 and 20 may include multiple layers. For example, the conventional pinned layer 12 and/or 20 may be a synthetic antiferromagnet (SAF) including magnetic layers antiferromagnetically coupled through thin conductive layers, such as Ru. In such a SAF, multiple magnetic layers interleaved with a thin layer of Ru may be used. In another embodiment, the coupling across the Ru layers can be ferromagnetic.
The conventional free layer 16 has a changeable magnetic moment 17. Although depicted as a simple layer, the conventional free layer 16 may also include multiple layers. For example, the conventional free layer 16 may be a synthetic layer including magnetic layers antiferromagnetically or ferromagnetically coupled through thin conductive layers, such as Ru. The pinned layers 12 and 20 and free layer 20 have their magnetic moments 13, 21, and 17, respectively, oriented perpendicular to the plane of the layers.
To switch the magnetic moment 17 of the conventional free layer 16, a current is driven perpendicular to plane (in the z-direction). When a sufficient current is driven from the conventional pinned layer 12 toward the pinned layer 20, the magnetic moment 17 of the conventional free layer 16 may switch to be parallel to the magnetic moment 21 of the conventional pinned layer 20. When a sufficient current is driven from the conventional pinned layer 20 toward the conventional pinned layer 12, the magnetic moment 17 of the free layer 16 may switch to be parallel to that of the pinned layer 12. The differences in magnetic configurations correspond to different magnetoresistance levels and thus different logical states (e.g. a logical “0” and a logical “1”) of the conventional MTJ 10.
FIG. 2 is a flow chart depicting a method 50 for setting the magnetic moments 13 and 21 of the pinned layers 12 and 20. Referring to FIGS. 1-2, for the purposes of the method 50, it is assumed that the pinned layer 12 has a higher coercivity than the pinned layer 20. Thus, a field, H1 shown in FIG. 1 that is in the desired direction of the magnetic moment 13 conventional pinned layer 12 is applied, via step 52. The magnitude of the field H1 is greater than the coercivities of both pinned layers 12 and 20. Thus, both magnetic moments 13 and 21 will be in the negative z direction while the field H1 is applied. This field is then removed. Thus, the magnetic moments 13 and 21 would be in the negative z direction after step 52 is performed.
Another field H2 is applied, via step 54. This field is in the positive z direction—the desired direction of the magnetic moment 21 of the conventional pinned layer 20. The magnitude of the field H2 is greater than the coercivity of the pinned layer 20 and less than the coercivity of the pinned layer 12. Thus, the magnetic field H2 does not disturb the state of the conventional pinned layer 12. However, the magnetic moment 21 of the conventional pinned layer 20 is switched to be in the positive z direction. Thus, the conventional pinned layers 12 and 20 are in a dual state (magnetic moments 13 and 21 antiparallel). In the dual state, the conventional free layer 17 may be more efficiently programmed using spin transfer. In addition, the field at the free layer 16 from the pinned layers 12 and 20 are in opposite directions and tend to cancel. As a result, the offset field at free layer 16 may be reduced.
Although the conventional MTJ 10 may be written using spin transfer and used in an STT-MRAM, there are drawbacks. There may be difficulties in setting the magnetic moments 13 and 21 of the pinned layers 12 and 20, respectively.
Accordingly, what is needed is a method and system for setting the magnetic moments of the pinned layers of magnetic junctions in the desired directions. The method and system described herein address such a need.